A method known in the industry as "hot-wire" filler material addition is utilized in various welding processes to increase the filler deposition rate over that which can be achieved with cold-wire addition. This "hot-wire" effect is achieved by applying additional power to preheat the filler (usually in the form of a continuously fed wire). The resulting benefits are to melt filler of a predetermined composition and size with a given arc current at a faster rate, or to melt filler of a larger size at a similar rate, or to increase both the feed rate and the filler size simultaneously for a maximum increase in the filler deposition rate.
"Hot-wire" filler addition is practiced in the known welding art with either alternating or direct current from an additional, electrically isolated power supply. This current is applied to the filler material by means of an electrical contact nozzle through which the filler is fed. In this design, the nozzle is not electrically connected in a parallel circuit to the power supply for the arc, and therefore the voltage of the nozzle is independent of the arc voltage. In a known configuration, the filler material is heated between the end of the conductive nozzle and the "grounded" workpiece.
In another design that is uncommon in the welding industry, the nozzle is electrically connected in a parallel circuit to the power supply for the arc, and therefore the voltage of the nozzle is dependent on the arc voltage. This variation is known as "shunted-arc" hot-wire addition. It is known to exist only with the geometry where the non-consumable electrode and the filler nozzle are both approximately perpendicular to the work surface, i.e., with "vertical" wire feed. This geometry is intended for robotic joining applications to avoid the problem where, if the nozzle were inclined to the workpiece, then an additional rotation of the torch assembly would be required to maintain the nozzle in the same orientation with respect to the travel direction for nonlinear joint path shapes.
The resistive heating in the filler occurs as the electrical power is dissipated in the length extending between the end of the contact tube and the point where it enters the molten pool. This predetermined length of filler extension is essentially an electrical resistor with a continuously replenished resistive element. The degree of heating is independently determined by the feed rate, the length of filler from the contact tube to the workpiece, and the applied voltage for a predetermined filler size and material type. The voltage drop across this heated length is dependent upon the these independently set parameters. The current through this heated length is typically a dependent parameter, controlled according to Ohm's Law.
One problem with hot-wire addition is that the degree of wire heating cannot be well controlled due to the fixed (predetermined) length of filler which is resistance heated with existing nozzle designs.
A second problem is that the actual aim position, relative to the electrode tip, of a highly preheated filler becomes variable in part depending on the degree to which it is preheated and correspondingly made "limp".
A third problem is the excessively wide groove required for joining relatively thick material, necessitated by the need for the nozzle to have sufficient cross-sectional area to conduct the desired electrical current.
A fourth problem is the energy loss in the electrode, which increases the cooling requirements of the electrode holder without adding any additional capability to thermally preheat the filler material.
A fifth problem for the single power supply (shunted-arc) hot-wire configuration is the need to easily convert to cold-wire feed and back to hot-wire feed without a high-current switch at the torch or a high-current cable leading from a remote switch to the torch.